Marker trait for types that may be used as SIMD vector elements.
SAFETY: This trait, when implemented, asserts the compiler can monomorphize
#[repr(simd)] structs with the marked type as an element.
Strictly, it is valid to impl if the vector will not be miscompiled.
Practically, it is user-unfriendly to impl it if the vector won’t compile,
even when no soundness guarantees are broken by allowing the user to try.
The mask element type corresponding to this element type.